NMRSTUDY OF POLY(VINYL CHLORIDE)
produced by the first four carbon atoms (a decrease in AH of 3.6 kcal/mole, a decrease in Ah'* of 9 eu/mole). Stated in a different manner, the inductive and steric effects produced by the Jirst four carbon atoms appear to be more than twice those produced by the next fourteen carbon atoms. It will be observed in column 4 of Table I1 that the free energies of activation at 120" for the decarboxylation of malonic acid and its n-alkyl derivatives are fairly constant and equal to about 31.1 kcal/mole. This means that at this temperature the rate constant for the decarboxylation of malonic acid apparently is not affected by the presence of an n-alkyl moiet,y regardless of chain length. The free energies of activation at 120" for the decarboxylation of oxalic acid (vapor) and oxanilic acid (melt) are slightly greater than those for the malonic acid group and also fairly constant (average 31.75 kcal/mole). This means that the rates of decar-
*
2525
boxylation of these two compounds are approximately equal at 120" and less than those for the malonic acid series. Substituting the free energies of activation at 120" for these two different groups of compounds in the absolute reaction rate equation5 enables the calculation of the rate constants to be made. For the malonic acid group, k1200 in sec-I turns out to be 0.00042, for the oxalic acid group, 0.00017. In other words, at 120°, malonic acid and its derivatives will suffer decarboxylation 2.5 times as fast as will oxalic acid (vapor) or oxanilic acid (melt). Figure 1 is a plot of the enthalpy us. entropy of activation for the two groups of compounds listed in Table
11. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Nuclear Magnetic Resonance Study of Poly(viny1 chloride)
by Kermit C. Ramey The Atlantic Refining Co., Research and Development Department, Glenolden, Pennsylvania (Received February 14,1966)
The high-resolution nuclear magnetic resonance spectra of poly(viny1 chloride) yielded, upon examination at 60 and 100 Mc/sec, information on the various tactic forms present. The spectra are discussed in terms of the various conflicting analyses which have previously been proposed and an alternate analysis is suggested. The resonance of the 0 protons is interpreted in terms of a triplet centered at r 7.75, corresponding to isotactic diads, and two overlapping triplets centered at r 7.91 and 7.95, corresponding to syndiotactic diads, which in turn are attributed to the three possible tetrad configurations.
Introduction High-resolut,ionnuclear magnetic resonance spectrescopy has proved to be an effective tool for studying the stereochemical configuration of vinyl polymers.'-5 I n Some cases' that Of poly(methyl (PMM),' and poly(viny1 methyl ether) (PVME),2 the
spectra yield an unambiguous determination of the cases, such that Of Poly-
tactic content* In Other
(1) F. A. Bovey and G. V. D. Tiers, Fortschr. Hochpolymer. Forsch., 3 , 139 (1963). (2) K. C. Ramey, N. D. Field, and I. Hasegawa, J. Polymer Sci., ~ 2 865 , (1964).
Volume 70, Number 8 August 1066
KERMITC. RAMEY
2526
(vinyl alcohol) (PVA),3 poly(viny1 acetate) (PVAC),~ and the spectra do not yield an unambiguous determination of the tacticity even though one or more of the groups attached to a central monomer unit may be sensitive to the stereochemical configuration and other techniques must be used to make an assignment. I n a few cases, particularly that of poly(viny1 chloride) (PVC),4+10 and poly(styrene) (PS),l*ll the spectra have not been satisfactorily analyzed in terms of triads and diads. The high-resolution nmr spectrum of PVC in solution in chlorobenzene at 160" was first reported by Johnsen.* He interpreted the 40-Mc/sec spectrum in terms of two overlapping triplets centered at r 7.78 and 7.96 for the p protons and a quintuplet centered at 5.53 for the CY protons. He tentatively assigned the two upfield triplets to meso and racemic (isotactic and syndiotactic diads) methylene groups with increasing field strength, respectively. Bovey, el uZ.,~ reported on the normal and decoupled spectra of PVC at 60 l\/lc/sec. Their results show two singlets for the resonance of the p protons decoupled from the CY protons, which appears to confirm the above interpretation of the methylene resonance, and similar results were obtained on deuterated samples.I2 The resonance of the CY proton decoupled from the p protons yielded three singlets, contrary to the previous report,s and a tentative assignment was made, that of i, h, and s triads with increasing field strength, respectively. Satohlo also reported on the decoupled spectra of PVC and the results are in good agreement with those reported by Bovey, et aL4 This analysis of the spectrum of PVC was, however, questioned by Tincher13 and he suggested that the methylene pro ton resonance could be analyzed in terms of an AB quartet for isotactic tliads and an A2 for the protons in syndiotactic diads, both of which were further complicated by coupling to the CY protons. More recently, he has concluded that the observed methylene resonance does not agree, except for the gross features, with either of the two previous analyses and has in turn suggested an alternate analysis in terms of chain branching. Still another analysis was indicated by the work of Yoshino and K ~ m i y a m a 'on ~ the spectrum of poly(a-cis-/3-d2vinyl chloride) which exhibits ten peaks for the /3 protons and they assigned these to the ten possible tetrad configxrations. They also reported on the spectra of poly(trans-p-dl- and -a-dl-vinyl chloride), both normal and decoupled and at 60 and 100 Mc/sec. Their results are not, however, completely satisfactory. The spectrum of poly(cu-dl-vinyl chloride) and the resonance of the /3 protons decoupled from the CY protons of PVC should be essentially the same, aside from the chemical shift difference caused by the presThe Journal of Physical Chemistry
ence of deuterium, and this does not appear to be the case. Their results14indicate that only the a-cis-pdz polymer spectrum permits the discrimination of tetrads and that the normal spectrum of the protons should be interpretable in terms of diads. Furthermore, the spectrum of the CY-& polymer14appears to be somewhat different from that reported by Tincherg for the same polymer.
Experimental Section The spectra were determined at 60 and 100 Mc/sec, on Models A-60 and HA-100 Varian spectrometers. The solutions contained approximately 10% (w/v) of polymer in solution in o-CsH4Clzand were run at 150". The sample of poly(methy1 methacrylate) was prepared from the free radical polymerization of methyl methacrylate in xylene at 80" using azobisisobutyronitrile. The sample of poly(viny1 chloride) is commercially available (Union Carbide QVNY). The spectra were calibrated by side bands produced by audiofrequency modulation of the magnetic field. -4Hewlett-Packard 2005 audiofrequency oscillator in conjunction with a Hewlett-Packard 5512A counter was used.
Discussion In Figure 1A is shown the 60-Mc/sec nmr spectrum of a commercial sample of PVC in solution in o-CeH4Clz as obtained at 150". The spectrum is essentially the same as that previously reported by Bovey, et uLj4 and 0thers,~'~0 and the upfield portion of the 0-proton resonance appears to be more complex than the downfield portion as previously noted by T i n ~ h e r . ~In Figure 1B is shown t,he corresponding spectrum of PVC as obtained at 100 Mc/sec under essentially the same conditions of temperature, solvent, and concentration. In both cases the resolution is approximately the same, being less than 1 cps as determined from the resonance (3) K.C.Ramey and N. D. Field, J. Polymer Sci., B3, 63 (1965). (4) F. A. Bovey, E. W. Anderson, D. C. Douglass, and J. A. Manson, J . Chem. Phys., 39, 1199 (1963). (5) K. C. Ramey and G. L. Statton, Makromol. Chem., 85, 287 (1965). (6) K.C. Ramey and N. D. Field, J. Polymer Sei., B3, 69 (1965). (7) K.C. Ramey, N. D. Field, and A. E. Borchert, ibid., A3, 2885 (1965). (8) U. Johnsen, ibid., 54, 56 (1961). (9) W.C. Tincher, Makromol. Chem., 85, 20 (1965). (10) S. Satoh, J. Polymer Sei., A2, 5221 (1964). (11) F. A. Bovey, F. P. Hood, E. W. Anderson, and L. C. Snyder, J. Chem. Phys., 42, 3900 (1965). (12) F. A. Bovey and G. V. D. Tiers, Chem. Ind. (London), 1826 (1962). (13) W.C. Tincher, paper presented before the Division of Polymer Chemistry a t the 142nd National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1962. (14)T. Yoshino and 3. Komiyama, J . PoZymeT Sei.,B3, 311 (1965).
NMRSTUDYOF POLY(VINYL CHLORIDE)
~
2527
~~
Table I: Species and Probabilities of Triads and Pentads, and Diads and Tetrads in Vinyl Polymer Chains Obeying Bernoulli Trial Statistics ( u = Probability of Isotactic Monomer Placement) a Substituents Designation
Isotactic,
8 Protons--
7
Probability
Designation
Probability
Isotactic, I or meso, m
i Diads
[
-
0
Syndiotactic, S or racemic, r
Syndiotactic, s iii
Isotactic diad
iIi or imi iIh or imh hIh or hmh
a3 two components 2 ~ 2 ( 1- a) two components a( 1 - a)2 two components
hSs or hrs
2a( 1 - a)z two components d ( 1 - U ) one component (1 - u)3 one component
[
Tetrads
diad
n .k
A
W 5
7
6
8
Y Figure 1. Nuclear magnetic resonance spectra of poly(viny1 chloride) in solution in o-CsHaCl2 as obtained a t 150": A, 500 cps sweep width a t 60 Mc/sec; B, portions of a 5Wcps sweep width at 100 Mchec.
of the internal reference tetramethylsilane. The 100Mc/sec spectrum of the p protons appears to confirm the conclusion reached by Tinehere that the two previously suggested analyses, based on equivalent p protons in syndiotactic diads and either equivalent or nonequivalent) p protons in isotactic diads, are not consistent with the observed spectrum. Furthermore, the spectrum appears to rule out branching as a major complicating factor; however, the effects of branching are difficult to predict. The use of higher frequencies for the nmr study of polymers is expected, in general, to simplify the spectrum and the interpretation thereof. However, in some cases the spectrum as obtained a t higher fre-
sSs of srs
quencies may be more complex as finer structure becomes observable. For the a substituent, one may expect to resolve pentads of monomer units, and as illustrated in Table I, the central monomer units in i, h, and s triads is expected to yield ten components. Similarly, for the p protons, one may expect to resolve tetrads of monomer units which should also yield ten components as illustrated in Table I. An attempt was made to interpret the normal resonance of the p protons of PVC in terms of the results obtained from the spectrum of the a-cis-p-d2 polymer. l 4 The agreement of calculated and observed spectra was, however, rather poor. In a recent report on the 60-Mc/sec nmr spectrum of poly(methy1 methacrylate), Bovey15 noted that the resonance of the protons corresponding to syndiotactic diads was split into three components and he assigned these to the three configurations in terms of tetrads, hrh, hrs, and srs, with increasing field strength, respectively. However, the p protons in isotactic diads and the a-methyl protons did not appear to be sensitive to the stereochemical configuration in terms of tetrads and pentads. Similar results are obtained at 100 Mc/ sec, as illustrated in Figure 2, which shows the nmr spectrum of poly(methy1 methacrylate) in solution in o-CBH&12 as obtained a t 150". The overlap of the downfield portion of the AB quartet corresponding to isotactic diads is indicated by the thatched lines in Figure 2, and could obscure the third component of the syndiotactic group that is observed at 60 Mc/sec.
(15)
F. A. Bovey,
t o be published.
Volume 70,Number 8 Auguat 1966
KERMIT C.RAMEY
2528
PMM
8
7
6
n
9
r
t
10
7.55
7.65
7.75
7.85
795
a05
7
Figure 4. A slow sweep (100-cps sweep width) of the 60-Mc/sec 8-proton resonance of poly(viny1 chloride) in solution in 0-C6H< as obt,ained at 150".
Figure 2. A lO0-Mc/sec nmr spectrum of poly(methy1 methacrylate) in solution in O-CeHaCh as obtained at 150".
B n
76
7 ;,
7,s
7.9
80
8.1
d Figure 3. A slow sweep (100-cps sweep width) of the 100-Mc/sec 8-proton resonance of poly(viny1 chloride) in solution in o-C6H&l~as obtained a t 150".
The two sharp lines at 7 7.78 and 7.85 are believed to be impurities. A slow sweep of the P-proton resonance of PVC as obtained at 100 R/lc/sec is shown in Figure 3. The spectrum is interpreted in terms of a triplet centered at 7 7.75 (JAX 6.8 cps) corresponding to isotactic diads and two triplets centered at 7 7.91 and 7.95 ( J A X = 6.8 cps) corresponding to syndiotactic diads. The resonance at 7 7.91 is tentatively assigned to an hrh configuration in terms of tetrads while the upfield component at 7 7.93 is tentatively assigned to the overlap of hrs and srs configurations. This assignment appears to be reasonable in view of the predominance of syndiotactic configurations. It is, nevertheless, open to question, since it is based primarily upon the most probable statistical distribution. The calculated p=5
The Journal of Physical Chemistry
I
7.5
.
.
7.7
.
.
7.9
,
I
83
Figure 5. Nuclear magnetic resonance spectra of the p protons of poly(viny1 chloride): A, normal; and B, decoupled from the CY protons.
proton resonance at 60 Mc/sec, as based on the above interpretation, is in fair agreement with the observed and the results are illustrated in Figure 4. The 100Mc/sec spectrum of poly(cr-dl-vinyl chloride) has been reported by Tinchers and shows a distinct shoulder on the upfield peak, which is consistent with the above interpretation. This analysis appears to be confirmed by the resonance of the P protons decoupled from the a protons as illustrated in Figure 5. The decoupled resonance clearly shows a singlet which corresponds to isotactic diads and two singlets which correspond to syndiotactic diads, contrary to the previous report.14 Although the resonance of the a protons at 100 Mc/ sec appears to be more complex than the 60 Mc/sec,
RADIOLYSIS OF FROZEN SOLUTIONS
2529
the resonance of the CY protons decoupled from the @ protons yielded only three singlets. An analysis based on the above results indicates that the resonance should appear more complex and the calculated and observed spectra are in fair agreement.
Radiolysis of Frozen Solutions. VI.
These results, which are quite similar to those obtained for PMM,16 indicate that only the p protons of PVC in syndiotactic diads are sensitive to the configuration in terms of tetrads and that the other groups may be interpreted in terms of diads and triads.
Electron and Excited Water
Reactions in Nitrate Ices
by Larry Kevan Department of Chemistry, University of Kansas, Lawrence, Kansas
(Received February 21, 1966)
~~
The y radiolysis of frozen nitrate solutions irradiated at 77°K has been studied. Nitrite yields were measured as a function of nitrate concentration, pH, radiation dose, and cation type. The data are compared with epr and aqueous solution results from which we conclude that NOz- is mainly produced by reactions of electrons and of electronically excited water with nitrate. The latter is responsible for the enhanced NOz- yields above pH 11. The results in aqueous and in frozen solutions show very close parallels and strongly imply that the same radiation-produced intermediates are formed and react similarly in both phases.
Introduction In the radiolysis of frozen nitrate solutions at 77°K the NO2 radical is observed directly by epr. Previous studies have shown that NO3- acts as an electron scavenger in frozen solutions.z However, from the effect of electron scavengers and the NO2 yield dependence on nitrate concentration it was proposed' that the trapped NOS arose from reaction of N03- with HzO+. This proposal probably received little serious consideration among aqueous solution radiation chemists. Recently though, the possibility of reactions of H20+in ices has been strongly supported by the studies of Moorthy and we is^.^ I n aqueous solution studies the solvated electron reacts with NO3- eventually to yield NOz-. The aqueous reaction is thought to involve an NO2 intermediate. To clarify the relationship between the reaction of NO,- with e,- (ern- denotes the mobile electron formed in ices) and with HzO+ in
frozen solutions and the aqueous nitrat.e solution results, we have studied the NOZ- yields produced upon melting irradiated, frozen nitrate solutions. A second goal of these chemical studies is to demonstrate the parallelism between aqueous solution radiolysis and frozen solution radiolysis. We further point out that information gained from the two approaches is largely complementary and serves to strengthen some conclusions and clarify others.
Experimental Section All chemicals were reagent grade. Water was triply distilled over alkaline permanganate and had pH 6. Oxygenated and nitrogenated solutions were prepared (1) L. Kevan, J . Phgs. Chem., 68, 2590 (1964). (2) L.Kevan, P. N. Moorthy, and J. Weiss, J . Am. Chem. SOC.,86, 771 (1964). ( 3 ) P.N.Moorthy and J. J. Weiss, J. Chem. Phys., 42,3127 (1965).
Volume 70, Number 8 August 1966
